1. Field of the Invention
The present invention relates to a power converter for driving an AC electric motor (induction motor, synchronous motor) utilizing an inverter, and more particularly to a power converter suitable for an electric motor requiring a wide range of speed change.
2. Description of the Prior Art
Ordinarily when a speed control of an AC motor is performed with an inverter as a driving power source, the inverter is classified into the following two types:
1. An inverter to the DC input side of which a smoothing reactor is connected and which functions as a current source to produce a rectangular output current. This is called a current type inverter. Since the direction of the current of this current type of inverter is always constant, the regenerative braking of the motor can easily be performed merely by reversing the polarity of the voltage to simplify the control circuit of the inverter itself. There is a further advantage that the motor can be stably started even from such a very low frequency as several Hz or a fraction of Hz. On the contrary, however, since the impedance as seen from the load side, i.e., the power source impedance is very high due to the fact that it functions as a power source, hunting is liable to occur at a steady running time which is the most serious problem to the operation of the motor. In particular, the hunting is liable to occur at such a high frequency of steady running as several hundreds of Hz. This phenomenon is more pronounced when a number of motors are running in parallel.
2. An inverter to the DC input side of which a smoothing capacitor is connected and which functions as a voltage source to produce a rectangular output voltage. This is called a voltage type inverter. Since this voltage type inverter functions as a voltage source, the power source impedance is very low, so that it has the advantage that unlike the current type inverter the hunting does not occur to enable the motor to run stably. On the contrary, however, the inrush current at the starting time of the motor cannot be suppressed to hunder a stable start of the motor. In particular, a stable start of the motor from such a very low frequency as several Hz or a fraction of Hz cannot be done.
Thus, when the speed control range of the motor, i.e., the output frequency range (the ratio of the maximum frequency to the minimum frequency) of the inverter is about several times, even the voltage type inverter can sufficiently satisfy the specification thereof. However, if the range is extended to about several tens of times to start the motor from such a very low frequency as several Hz or a fraction of Hz (minimum frequency), a stable start cannot be done with the voltage type inverter. In this case the current type inverter is preferable also from the standpoint of suppressing the inrush current at the starting time. However, the current type inverter has the difficulty in the stability of the steady running, while the voltage type inverter is preferable from the standpoint of the stability of the steady running.
Consequently, in the past, when the motor was to be run at such a speed control range as described above, two inverters of the current type and the voltage type had to be prepared so that they could be changed-over as required. As a result, the system became expensive.